Permanent soil and subsoil measurement probe

ABSTRACT

A device for monitoring an underground formation containing a fluid such as CO 2  or methane comprising a measuring cell (CM) arranged in a cavity, analysis means (MA) arranged on the surface and sealed connection means (ML) connecting measuring cell (CM) to analysis means (MA). The measuring cell comprises two chambers (CH 1 , CH 2 ) that can sealingly communicate with one another. The First chamber (CH 1 ) comprises a plurality of orifices (OR) allowing passage of the fluid into first chamber (CH 1 ), at least two inner electrodes (EI) and fluid circulation means (MC). Second chamber (CH 2 ) is impervious to the fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to French Application No. 15/58.326, which application is incorporated herein by reference by its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to monitoring the impact, on the soil and the subsoil, of anthropic activities involving or generating fluids, notably gases.

More particularly, the present invention can concern the monitoring of geological storage sites for fluids such as carbon dioxide (CO₂) or methane.

Description of the Prior Art

Examples of such activities are waste storage (nuclear waste for example), fluid transport networks (pipelines), underground fluid storage (fuels, gas), accidental soil pollution, pollution related to non-accidental industrial activities, geological fluid production (hydrocarbons, deep waters, geothermics).

It may be of interesting to cross-reference gas exchange measurements with other types of physical measurements in order to refine, through cross-interpretation, the understanding of fluid movements in an underground formation. Ideally, to guarantee representativity of the various parameters measured and to limit environmental disturbance, these measurements need to be performed simultaneously and in the same place. Ideally, a measuring device for monitoring an underground formation containing a fluid must be able to stay in its measurement environment as long as necessary, in continuous connection or not with measurement analysis and/or supply means, optionally remote. Environmental monitoring of the impact of anthropic activities on the soil and the subsoil requires tools enabling normalized representative measurements over relatively long periods of time (years, decades).

Measuring gas exchanges in the soil and the subsoil is one aspect of environmental monitoring, which relates both to soil performance and to possible gas migration through the soil and subsoil. Within the context of measuring gas exchanges in the soil and subsoil, it may be desirable to have measuring devices accounting for a process chain ranging from signal or matter sampling through significant parameter measurement to measurement processing and uncertainty analysis. In order to ensure measurement reliability and representativity, it is desirable that the implemen-tation of this process chain has the least possible impact on the environment to be characterized. The physical interface of a measuring device therefore is a critical element that can concentrate alone the risks of environmental disturbance and the representativity of the information gathered.

The following documents are mentioned in the description hereafter.

-   Noemie Taquet, Jacques Pironon, Philippe De Donato, Hervc Lucas,     Odile Barres (2013). Efficiency of Combined FTIR and Raman     Spectrometry for Online Quantification of Soil Gases: Application to     the monitoring of Carbon Dioxide Storage Sites. International     Journal of Greenhouse Gas Control, 12, 359-371. -   Thomas M. Christy (1998). A Permeable Membrane Sensor for the     Detection of Volatile Compounds in Soil. Symposium on the     Application of Geophysics to Engineering and Environmental Problems     1998: pp. 65-72.

Document (Taquet et al., 2013) describes a device designed for collection of information relative to soil gas, collection being achieved in a (mini) wellbore. More precisely, this document describes a gas sample chamber having of the base of a well, insulated from the wellbore atmosphere by an inflatable cushion referred to as packer. The gas is extracted from the chamber by a pump, then it flows through a series of analyzers and sensors prior to being reinjected in the vicinity of the sample chamber through stainless steel tubes. Furthermore, a temperature measurement is performed at the sample chamber. Surface equipments allow measurement of the gas composition, the gas pressure and flow rate, and to maintain the packer pressure. The sample chamber is equipped with a semi-permeable membrane enabling the gaseous species to be allowed into the tubes, but not liquid water. Due to the packer installation and pressure maintenance, this system is difficult to implement. Besides, there is no description of characteristics allowing ensuring the non-disturbance of the measurement environment or the resistance to corrosion over time.

The GEOPROBE Company that develops systems dedicated to soil gas sampling is also known. These systems, mode of steel, allow rapid drilling of different types of soil and recovering the gases present in the bottomhole. More particularly, the Geoprobe MIP probes (see (Christy, 1998) for example) allow driving the soil gas upwards via the circulation of a carrier gas and to measure the electrical conductivity of the soil. These probes are made of steel with measurements being performed while drilling. They are intended for specific applications and are not designed for long-term monitoring of a storage site. Furthermore, these probes require injection of a carrier gas other than the sampled gas, and they do not reinject the sampled gas, which does not guarantee the non-disturbance of the measurement environment.

Thus, none of the devices and systems according to the prior art guarantees both non-disturbance of the measurement environment, measurement of the gas content, measurement of the electrical properties of soils and suitability, owing to their operation and materials, for long-term monitoring of a formation containing a fluid.

The present invention describes an underground device for monitoring an underground formation containing a fluid, allowing at least sampling and analysis of the fluid present in the formation, and measurement of the electrical properties of the formation being studied. The probe according to the invention comprises a measuring cell made up of at least two chambers, a first chamber is located where the measurements are actually performed and a second chamber is for protection of the connection means between the measuring instruments and the means for analyzing these measurements. The measurement analysis means can be remote means arranged on the surface for example, and the connection means can comprise a corrosion-resistant sealed sheath.

Thus, the device according to the invention is suited for long-term monitoring of an underground formation containing a fluid, such as a geological gas storage site.

SUMMARY OF THE INVENTION

The present invention thus is a device for monitoring an underground formation containing a fluid. The device comprises at least one measuring cell intended to be arranged in a cavity provided in the underground formation, analysis means intended to be arranged on the surface, connection means connecting the measuring cell to the analysis means. The cell comprises at least a first chamber, a second chamber impervious to the fluid, and at least three inner connectors sealingly connecting the first chamber to the second chamber with the connection means sealingly connecting the measuring cell to the analysis means, and the connection means comprising sealed protection means, the first chamber comprises:

-   -   a plurality of orifices allowing passage of the fluid into the         first chamber,     -   at least two inner electrodes cooperating with at least two of         the inner connectors, the inner electrodes being connected to         the analysis means through the connection means and the         connection means cooperates with the inner connectors,     -   fluid circulation means connected to the analysis means while         cooperating with at least one of the connectors and with the         connection means.

According to one embodiment of the invention, the device can additionally comprise at least two outer electrodes, two inner connectors connecting the two chambers, two sealed outer connectors arranged on at least one wall of the first chamber, the wall being in contact with the formation, the outer electrodes cooperating with the outer connectors, the outer electrodes being connected to the analysis means through the connection means, the connection means cooperating with the two inner connectors and the two outer connectors.

According to an embodiment of the invention, the connection means can comprise power supply means supplying the electrodes, and the sealed protection means comprise a sealed sheath protecting at least the circulation means and the power supply means.

Advantageously, the first chamber can be filled with a permeable porous material for which petrophysical and electrical properties are known.

Preferably, the petrophysical properties can be at least porosity and permeability, and the electrical properties can be at least electrical conductivity.

According to an embodiment of the invention, the connection means, the chambers and the connectors can be made of PTFE.

According to an embodiment of the invention, the analysis means can at least comprise a fluid analyzer and/or a resistivity meter.

According to an embodiment of the invention, the fluid circulation means can comprise at least a pipe, a fluid suction system and a fluid reflux system.

Advantageously, the fluid circulation means can comprise at least two pipes, a fluid suction system and a fluid reflux system.

Furthermore, the invention relates to a use of the device according to the invention for monitoring a geological storage site for a gas such as CO₂ or methane.

Preferably, a calibration stage can be carried out prior to injecting gas into the geological storage site when using the device according to the invention for monitoring a geological gas storage site.

Besides, the invention relates to a method for monitoring an underground formation containing a fluid, wherein at least one device for monitoring an underground formation containing a fluid according to the invention is used and wherein at least the following stages are carried out:

-   -   drilling a cavity for receiving the measuring cell of said         device;     -   connecting the measuring cell to the connection means of the         device,     -   arranging the measuring cell within the cavity thus formed and         arranging the analysis means of the device on the surface,     -   connecting the analysis means to the connection means of the         device,     -   performing measurements using the measuring cell and analyzing         the measurements using the analysis means.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the method according to the invention will be clear from reading the description hereafter of embodiments given by way of non limitative example, with reference to the accompanying figures wherein:

FIGS. 1 to 5 diagrammatically illustrate in cross-sectional view various embodiments of the device for monitoring an underground formation containing a fluid according to the invention;

FIG. 6A illustrates an external view of an embodiment of the measuring cell of the device for monitoring an underground formation containing a fluid according to the invention, and FIG. 6B corresponds to a vertical section along the measuring cell of the device according to the invention shown in FIG. 6A.

FIG. 7A (FIG. 7B respectively) shows the evolution of the electrical resistivity measured by inner electrodes (outer electrodes respectively) as a function of the water saturation, for three different assembly configurations of the measuring cell of the device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a device for monitoring an underground formation containing a fluid. The fluid can be gas, such as CO₂ or methane, or a liquid such as a liquid hydrocarbon phase. The fluid may have been, not restrictively, intentionally stored in the formation studied (for example in the case of geological CO₂ storage) or it can result from a degradation of products stored in the formation studied (for example in the case of geological waste storage), or it can be a natural fluid geologically trapped in a subsoil formation.

The device according to the invention comprises a measuring cell to be arranged in a cavity provided in the underground formation being considered. The cavity may have been formed by drilling for example, to the size of the measuring cell, so that the cell is in direct contact with the formation. According to the invention, the measuring cell has at least a first chamber and a second chamber. The measurements are actually performed in the first chamber using measuring instruments, and the second chamber serves as a protection for connection means connecting the measuring instruments to means for analyzing these measurements. The measurement analysis means can be arranged on the surface for example. According to the invention, the connection means sealingly connect the measuring cell to the analysis means and the connection means comprise sealed protection means.

Furthermore, at least three inner connectors sealingly connect the first chamber to the second chamber, these connectors allowing passage of the measurement.

Besides, the first chamber comprises at least:

-   -   a plurality of orifices allowing passage of the fluid into the         first chamber,     -   at least two inner electrodes cooperating with at least two of         the inner connectors, the inner electrodes being connected to         the analysis means through the connection means, the connection         means cooperating with the inner connectors, the electrodes         allowing to perform an electrical measurement relative to the         fluid present in the first chamber;     -   fluid circulation means, connected to the analysis means while         cooperating with at least one of the connectors and with the         connection means.

FIG. 1 shows a non limitative example of embodiment of the device according to the invention, and the various elements of the device according to the invention can be arranged differently in relation to one another and in the absolute. FIGS. 2 to 5 show by way of non limitative and non exhaustive example other variant embodiments of the device according to the invention. FIGS. 1 to 5 correspond to a section along a vertical plane of the device for monitoring an underground formation containing a fluid.

The device according to the invention comprises a measuring cell CM intended to be arranged in a cavity provided in the underground formation considered. The cavity may have been formed by drilling for example, to the size of measuring cell CM, so that cell CM is in direct contact with the formation. According to the invention, measuring cell CM has at least a first chamber CH1 and a second chamber CH2. Advantageously, the second chamber CH2 is positioned above the first chamber CH1 as shown by way of non limitative example in FIG. 1.

According to the invention, the device is equipped with analysis means MA that can be remote means arranged on the surface for example and with connection means ML sealingly connecting the measuring cell CM to analysis means MA. According to the invention, the connection means are themselves protected by sealed protection means.

According to the invention, the wall of first chamber CH1 is pierced with a plurality of orifices OR allowing sampling of the fluid present in the formation from the inner volume of first chamber CH1. According to the invention, second chamber CH2 is impervious to the fluid. Besides, according to the invention, at least three inner connectors CI sealingly connect first chamber CH1 to second chamber CH2.

According to the invention, first chamber CH1 additionally comprises at least two inner electrodes EI cooperating with at least two of the inner connectors CI of the cell, inner electrodes EI being connected to analysis means MA through connection means ML, connection means ML cooperating with inner connectors CI.

According to the invention, first chamber CH1 also comprises circulation means MC for the fluid collected in first chamber CH1, the circulation means MC for the fluid collected in first chamber CH1 being connected to the analysis means MA while cooperating with at least one of the connectors CI and with the connection means ML.

According to the invention, the connection means provide a sealed connection between the measuring cell and the analysis means, thus enabling transmission without loss of the measurements performed by the measuring cell to the analysis means.

Furthermore, the connection means are protected by sealed protection means. This guarantees the non-alteration over time of the connection means themselves, and thus contributes to the durability of the device according to the invention and to the suitability of the device according to the invention to the long-term monitoring of an underground formation containing a fluid.

The imperviousness of the second chamber allows protection of the part of the elements of the device according to the invention positioned in this second chamber (at least connection means ML as shown in FIG. 1), and thus to contribute to the non-alteration over time of the part of the elements in question, and therefore to the suitability of the device according to the invention to the long-term monitoring of an underground formation containing a fluid.

The imperviousness of the inner connectors is intended to prevent passage of part of the fluid collected in the first chamber to the second chamber, and thus erroneous transmission to the analysis means of the amount of fluid collected, but also to preserve the integrity of the second chamber and of the part of the measuring elements of the device according to the invention (at least connection means ML as shown in FIG. 1) that it contains.

According to an embodiment of the invention, the connection means comprise power supply means for the electrodes, and the sealed protection means comprise a sealed sheath protecting at least the fluid circulation means and the power supply means. FIG. 2 shows by way of non limitative example a variant of this embodiment.

For this variant, the sealed protection means comprise a sealed sheath GA connecting the top of measuring cell CM to analysis means MA, and the power supply means correspond to electric cables CEL, electric cables CEL cooperating with inner connectors CI and inner connectors CI cooperating with two inner electrodes EI. According to this embodiment, a first part of electric cables CEL runs through second chamber CH2 and a second part (not shown), inserted in sheath GA, connects the top of the second chamber to analysis means MA. According to this embodiment of the invention, an opening is provided in the top of second chamber CH2, this opening cooperating with sheath GA. Still according to this embodiment of the invention shown in FIG. 2, the fluid taken from first chamber CH1 of measuring cell CM is taken up by fluid circulation means MC, a first port thereof being located in first chamber CH1, a second part runs through at least one of the inner connectors CI, a third part runs through second chamber CH2 and a fourth part (not shown) which is inserted in sheath GA, connects the top of second chamber CH2 to analysis means MA. According to an embodiment of the invention, the electric cables are insulated with a silicone sheath and the sheath is made of polymer with a helical steel armour.

FIGS. 3 to 5, which illustrate other embodiments of the present invention, show hereafter by way of non limitative and non exhaustive example connection means comprising a sheath GA, electric cables CEL and remote analysis means MA, the part of the electric cables and the part of circulation means MC arranged outside measuring cell CM being inserted in sheath GA. Any other layout and configuration of the connection means allowing measuring cell CM to be connected to analysis means MA is however possible, and analysis means MA can furthermore be remote means or not.

According to an embodiment of the present invention, the orifices of the first chamber can be:

(1) covered with a semi-permeable/hydrophobic membrane (of 0.2 μm-diameter hydrophobic polypropylene type for example) so as to prevent water sampling in the first chamber and water upflow in the fluid circulation means and towards the analysis means. This embodiment is particularly suited when the fluid of interest is a gas and in the case of an area where submergence is considered (flood-risk area, high water table fluctuations, heavy rains and bad drainage conditions), or

(2) covered with a geotextile, geotextiles having the property of being hydrophilic (which ensures good hydric and electric coupling between the inner volume of the first chamber and the formation) and anti-particulate (thus preventing fine particles from entering the inner volume of the first chamber). This embodiment can be preferably used in well drained areas, far from waterways, out of the reach of the water table and submergence risk free, or

(3) free, i.e. not covered with any material. This embodiment allows maximum connectivity of the inner volume of the first chamber with the outside medium, and thus to measure fast gas composition fluctuations.

According to a particular embodiment of the present invention, the first chamber and the second chamber are made of polytetrafluoroethylene (PTFE) so as to ensure a good durability for measuring cell CM and a minimum chemical effect on the close environment.

According to a non limitative embodiment of the present invention shown in FIG. 3, the fluid circulation means comprise at least a tube T, a suction system ASP and a reflux system REF. Suction system ASP allows the fluid sampled in first chamber CH1 to be sucked into at least one inner connector CI, then into tube T, and to be transferred to analysis means MA. Reflux system REF allows the fluid, once collected and analyzed by analysis means MA, to be refluxed to the formation by passing again through the tube T, the inner connector CI, the first chamber CH1, then the orifices OR of first chamber CH1. By sampling the fluid and reinjecting it into the formation, this embodiment allows significant limitation of measurement environment disturbance, which is suitable for long-term monitoring of a site containing a fluid. Preferably for example, suction system ASP and reflux system REF are remote systems arranged on the formation surface. The embodiment of the invention shown in FIG. 3 comprises connection means including a sheath GA, electric cables CEL and remote analysis means MA, the part of electric cables CEL and the part of tube T outside measuring cell CM being inserted in sheath GA. Any other layout and configuration of the connection means and any other method of cooperation of these connection means with the fluid circulation means are however possible, and the analysis means can furthermore be remote means or not.

According to a non limitative embodiment of the invention shown in FIG. 4, the fluid circulation means comprise two tubes TA and TR, tube TA being connected to suction system ASP, and tube TR being connected to reflux system REF. Preferably, tube TR penetrates deep into the inner volume of first chamber CH1 while the termination of tube TA in first chamber CH1 substantially corresponds to the base of connector CI with which it cooperates. This configuration allows maximizing the distance D between the suction point and the reflux point, and thus maximizing stirring of the gas between the two tubes. Thus, the gas composition variations in the cell are more finely detectable. Preferably, distance D corresponds to 70% of the height of first chamber CH1. The embodiment of the invention shown in FIG. 4 comprises connection means including a sheath GA, electric cables CEL, remote analysis means MA, the part of electric cables CEL and the parts of tubes TA and TR outside measuring cell CM being inserted in sheath GA. Any other layout and configuration of the connection means and any other method of cooperation of these connection means with the fluid circulation means are however possible, and the analysis means can furthermore be remote means or not.

According to an embodiment of the invention, the analysis means comprise at least one fluid analyzer, which can be a remote analyzer arranged on the surface or not. The fluid analyzer allows detection and quantification (concentration assessment for example) of at least one type of fluid. Preferably, the fluid analyzer allows at least detection and quantification of the fluid injected into the formation.

According to an embodiment of the invention, the inner electrodes can be used for electrical resistivity measurements in the medium. According to this embodiment, at least part of the inner volume of the first chamber is advantageously filled with a porous reference material (preferably permeable to the fluid collected through the orifices) so that at least part of inner electrodes EI is in contact with this porous material. A porous reference material is understood to be a material whose petrophysical and electrical properties are known. Preferably, the petrophysical properties of the porous reference medium are at least porosity and permeability. Preferably, the electrical properties of the porous reference medium are at least the electrical conductivity. Advantageously, the porous material can be quartz sand. Thus, the fluid collected in the first chamber can lodge itself in the pores of the porous material, and a resistivity measurement, through the inner electrodes, in contact with this at least partly fluid-saturated material, is performed.

More preferably, the analysis means comprise at least one resistivity meter, remote and arranged on the surface or not. The connection means can then include electric cables allowing connection of the inner connectors cooperating with the electrodes internal to the resistivity meter.

According to a non limitative embodiment of the invention illustrated in FIG. 5 (the fluid circulation means are not shown in this figure for simplification reasons), at least two sealed outer connectors CE are positioned on one of the walls of first chamber CH1 in contact with the outside of measuring cell CM, outer connectors CE cooperating with at least two outer electrodes EE. Furthermore, according to this embodiment, the device also comprises at least two additional inner connectors CI.

The additional inner connectors CI allow passage of connection means (such as electric cables CEL shown in FIG. 5) between the outer connectors CE and the analysis means MA. Advantageously, the wall on which outer connectors CE are arranged is in contact with the formation of interest. Thus, outer electrodes EE running through this wall via outer connectors CE allow electrical measurements to be performed within the formation studied. Advantageously, analysis means MA comprise at least one resistivity meter and the electrical measurements performed by means of outer electrodes EE concern an electrical resistivity measurement in the formation of interest. According to a particular embodiment of the present invention, electrical resistivity measurements are performed using the at least two inner electrodes EI and the at least two outer electrodes EE positioned as described above. Thus, inner electrodes EI allow calibration of the resistivity measurement on a known simple porous medium (such as quartz sand) and therefore to serve as a reference in relation to the resistivity measurements performed with outer electrodes EE that are subject to more unknowns regarding the porous medium where they are inserted (mineralogical composition, porosity, grain size). Notably, this embodiment can be useful for quantifying and monitoring a potential resistivity measurement drift related to a property alteration of the inner and/or outer electrodes. The embodiment of the invention illustrated in FIG. 5 comprises connection means including a sheath GA, electric cables CEL, remote analysis means MA. The part of the electric cables CEL outside measuring cell CM is inserted in sheath GA. Any other layout and configuration of the connection means and any other cooperation of these connection means with fluid circulation means are possible, and the analysis means can furthermore be remote means or not. According to another embodiment of the invention, the inner electrodes and/or the outer electrodes are used for performing time domain reflectometry (TDR) measurements. According to this embodiment, the analysis means comprise a time domain reflectometer and the inner and/or outer electrodes are connected through connection means (preferably electric cables) to the time domain reflectometer. Such an embodiment allows quantifying the dielectric constant of the formation being studied, itself to be proportional to the water saturation of the formation being studied.

According to an embodiment of the present invention, the device furthermore comprises a temperature probe communicating with the inner volume of the first chamber through an inner connector. Thus, the device also allows measuring the temperature within the formation being studied. This temperature measurement can be useful for determining thermal conductivities in the formation being studied, by comparison with a surface temperature measurement, and for correcting the electrical resistivity measurements performed by the inner and/or outer electrodes biased by the effects of temperature on the medium conductivity.

According to an embodiment of the present invention, the device according to the invention also comprises a soil moisture measuring means. Such measurements can indeed allow calibration of the electrical measurements performed by the inner and outer electrodes regarding the soil moisture rate variations.

Variants

Variants of the present invention comprising elements enabling automated measurements over time are presented hereafter. The variants described below can be combined, alone or in combination, with any one of the embodiments described above.

According to an embodiment of the present invention, the device furthermore comprises an automaton allowing preprogramming of the measurements to be performed, whether of electrical, geochemical or temperature type. The automaton can for example allow defining a geochemical measurement sequencing by triggering successively with time, according to a given periodicity, sampling of the fluid, as well as transfer and analysis of this fluid. Likewise, the automaton can allow triggering electrical measurements with a certain periodicity, according to some parameters (number of electrodes involved in the measurement, electric current supplied, etc.).

According to an embodiment of the present invention, the device also comprises a data collector (such as the DT85GLM model marketed by the DIMELCO Company for example) and data transmission means. The data collector allows to collect the measurements analyzed in the analysis means and to transmit them in real time through the data transmission means.

According to an embodiment of the present invention, the collected data transmission means are remote transmission means (modem enabling internet connection for example). Preferably, the means for transmitting the data collected by the collector consist of a 3G modem.

Thus, the device according to the invention allows the data collected on the site to be transmitted automatically and in real time to a specialist who will be able to make ad hoc decisions in case of abnormal measurements performed in situ by the device.

According to an embodiment of the present invention, the data collector allows alert trigger points to be taken into account and it is able to trigger an alert. Thus, if an amount of fluid above a given threshold set by the specialist is detected through the fluid analyzer, the data collector is able to send an alert, for example to a specialist or to the authorities, through an email message, on audible warning, etc.

According to an embodiment of the present invention, the power supply of the device according to the invention is provided by a solar panel and it is connected to a battery.

According to another embodiment of the present invention, at least the analysis means, the data collector and possibly the automaton are protected on the surface in a sealed shelter.

According to an embodiment wherein the device of the invention comprises an automaton, a data collector and collected data remote transmission means, the device thus formed can allow performing measurements in an automatic and preprogrammed manner, to analyze these measurements using the analysis means, to collect the results of these measurements through the collector, then to transmit them to a potentially distant specialist through the remote transmission means. A device comprising such elements is indeed suited for long-term permanent monitoring of a site containing a fluid whose (chemical and/or spatial) evolution needs to be monitored.

Use of the Invention

The invention also concerns the use of the device according to the invention for monitoring on underground formation containing a fluid.

The invention can more particularly concern the use of the device according to the invention for monitoring a geological storage site for a gas such as carbon dioxide (CO₂) or methane.

Preferably, using the device according to the invention for monitoring an underground formation containing a fluid requires a device calibration stage prior to the monitoring phase proper. In order to guarantee characterization of the impact on the environment of the soil and subsoil of a geological storage site, characterization prior to the industrial activity in question needs to be done in order to define the reference state of the environment (also referred to as base line). The environmental monitoring tools must therefore provide for characterization of the natural environment, the spatial heterogeneities and the temporal variability thereof. It must also be possible to subsequently use the monitoring tools to measure the impact of an activity on the basis of a previously defined reference state.

Once installed on site, the device according to the invention can be used in continuous and/or permanent acquisition mode, or for chronic measurements. In the case of permanent measurement, analysis means MA can be connected to measuring cell CM by permanent connection means. In the case of chronic measurement, the connection means can be removed and must allow analysis means MA to be temporarily disengaged from measuring cell CM, so as to be able to protect these analysis means MA between two measurements.

Moreover, the invention concerns a method of monitoring an underground formation containing a fluid by means of a device for monitoring an underground formation containing a fluid according to any one of the variants described above, and comprising at least the following stages:

-   -   drilling a cavity intended to receive the measuring cell of the         device,     -   connecting the measuring cell to the connection means of the         device,     -   arranging the measuring cell within the cavity thus formed and         arranging the analysis means of the device on the surface.     -   connecting the analysis means to the connection means of the         device,     -   performing measurements using the measuring cell and analyzing         the measurements using the analysis means.

APPLICATION EXAMPLE

The features and advantages of the method according to the invention will be clear from reading the application example hereafter.

The application example in question relates to the monitoring of an underground formation into which CO₂ has been injected, monitoring being achieved according to a particular variant of the device of the invention illustrated in FIGS. 6A and 6B. More precisely. FIG. 6A shows an external view of measuring cell CM according to the embodiment of the invention in question, and FIG. 6B shows a vertical section along said measuring cell CM according to the same embodiment. As FIG. 6B corresponds to a section in the volume of measuring cell CM. All of the elements making up the embodiment in question ore not shown.

Thus, the device as implemented for this application of the device according to the invention is characterized by:

-   -   a measuring cell CM made up of a first semi-open body 01, of         cylindrical revolution, the opening of the first body being         oriented upwards, the first body being provided with a cover 02         so as to form a closed first chamber CH1, cover 02 being         surmounted by a second semi-open body 03, of cylindrical         revolution, the opening of the second body being oriented         downwards, so as to form, with the cover 02, a closed second         chamber CH2. Thus, according to this implementation example,         cover 02 forms both the upper wall of first chamber CH1 and the         lower wall of second chamber CH2 of the cell. According to this         example, first body 01, cover 02 and second body 03 tightly fit         into one another through the agency of silicone doughnut rings,         and they ore secured by stainless steel pins 04 press-fitted in         orifices provided for that purpose. According to the same         implementation mode, the vertical wall of the first chamber is         pierced with oblong orifices OR and the basal wall is pierced         with circular orifices OR to allow passage of the gas present in         the formation of interest into the inner volume of first chamber         CH1. Besides, the top of second chamber CH2 comprises an opening         06 allowing at least passage of connection means ML. Still         according to this example, the two bodies and the cover are made         of PTFE so as to guarantee the durability of measuring cell CM.         The chambers thus formed are approximately 100 mm in diameter         and 100 mm high each,     -   five orifices pierce cover 02 of measuring cell CM with cover 02         corresponding both to the upper wall of first chamber CH1 and to         the lower wall of second chamber CH2, to allow communication         between the two chambers through inner connectors CI,     -   two orifices pierce the lower wall of first chamber CH1 to allow         passage of outer electrodes EE through outer connectors CE;     -   an inner volume of first chamber CH1 is 90% filled with quartz         sand of grain size 300 μm. This filling allows having a 2         cm-high gas overhead at the top of the inner volume of first         chamber CH1;     -   outer walls of measuring cell CM (including the orifices that         pierce the walls of first chamber CH1) covered with a geotextile         glued with a silicone-based glue. In addition to providing good         hydric and electric coupling between the inside of first chamber         CH1 and the formation, and preventing passage of particles into         the inner volume of first chamber CH1, the use of geotextile         also allows keeping the quartz sand within first chamber CH;     -   four electrodes (not shown) made of tungsten guarantee good         electrical conductivity and excellent corrosion resistance. More         particularly, the electrodes are crimped on adapters made up, on         one side, of a ⅛″ fitting of Swagelok® double ring type and, on         the other side, of a ⅛″ male NPT type fitting. Two of them         (referred to as inner electrodes) are screwed in two of the         tapped Teflon® orifices allowing communication between the first         CH1 and the second CH2 chamber. The other two electrodes         (referred to as outer electrodes) are screwed in the two         orifices allowing communication of first chamber CH1 with the         formation being studied. An inner connector CI or an outer         connector CE according to the invention is thus made up of a ⅛″         fitting of Swagelok® double ring type and a ⅛″ male NPT type         fitting. The inner CI and outer CE connectors associated with         inner electrodes and outer electrodes respectively comprise         electrical tin connectors allowing connection (through crimping)         of the electrodes to the electric cables cooperating with the         connectors CI, CE respectively. At least the part of the         electric cables located in first chamber CH1 and the electrical         connectors of outer connectors CE are electrically insulated         from the inner volume of first chamber CH1 by means of a         heat-shrink sheath. The electric cables associated with the         outer electrodes rise towards the analysis means by passing         through second chamber CH2 via two of the orifices pierced in         the cover of measuring cell CM, the passage being sealed by         means of a silicone-based glue,     -   fluid circulation means (not shown) comprising a fluid suction         means and a fluid reflux means, with each one comprising two         tubes, aspiration and reflux tubes respectively. The tubes of         the fluid circulation means are standard stainless steel         thin-rim ⅛″ tubes (1 mm (0.04″) inside diameter). They are         crimped on inner connectors CI made up, on one side, of a ⅛″         fitting of Swagelok® double ring type and, on the other side, of         a ⅛″ male NPT type fitting, then directly screwed in the tapped         Teflon® orifices allowing communication between the two         chambers. The suction tube ends at the top of first chamber CH1.         The reflux tube runs through the cover and first chamber CH1, up         to one centimeter above the basal wall of chamber CH1,     -   a temperature probe (not shown) of PT100 type (marketed by the         Prosensor Company), crimped through an inner connector CI made         up, on one side, of a ⅛″ fitting of Swagelok® double ring type         and, on the other side, of a ⅛″ male NPT type fitting, directly         screwed in the tapped Teflon® orifices allowing communication         between the two chambers. This temperature probe is a         feed-through probe that runs halfway down first chamber CH1. It         is supplied by an electric cable,     -   connection means (not shown) comprising a protection sheath,         armoured and flexible, connected to the top of second chamber         CH2 by two nuts on either side. The sheath rotates freely about         its principal axis, at the connection with the top of second         chamber CH2, so as to limit stresses thereon. The four electric         cables of the electrodes, the electric cable of the temperature         probe and the two tubes of the fluid circulation means run         through opening 06 provided in the top of second chamber CH2 and         the sheath prior to joining the remote analysis means. The         sheath is five meters long. The electric cables and the tubes         are six meters long, allowing direct connection with the remote         analysis means at the surface,     -   remote analysis means comprising a resistivity meter of Syscal         Junior 24 (Iris Instruments®) type and a gas analyzer allowing         to detect and to quantify CO₂, such as the LI-820 detector         marketed by the LI-COR Company for example.

The device according to the invention as described above was installed in a cavity provided by drilling in the formation studied.

100-mm holes were drilled with an auger bit and the soil samples were carefully collected for each 10-cm depth interval. Three holes were drilled at depths of: (1) 180 cm (2) 120 cm (3) 60 cm. The soil samples were analyzed in the laboratory to define their proportion of total organic carbon, mineral carbon, the major minerals distribution (clays, tectosilicates and carbonates), the porosity and the grain size distribution.

A fraction of the soil corresponding to the base of the hole is coarsely screened in order to remove the stones and the roots. This screened soil is placed in the bottom of the hole intended to receive the device according to the invention in order to fill again the remaining last 20 centimeters. Measuring cell CM is manually lowered by pushing it through armoured sheath GA and by guiding the verticality of measuring cell CM using a pole. Once contact is made with the base of the hole, measuring cell CM is driven in using the pole while ensuring that outer electrodes EE penetrate in the base of the hole by controlling the insertion distance. Once measuring cell CM in place, the hole is filled in with sand (100-μm diameter) until the cell is covered therewith, so as to guarantee good mechanical coupling of the porous media between the inside of measuring cell CM and the formation studied. The hole is then flooded with fresh water in order to rework the porous medium, to fill in any space left unfilled and to maximize the hydric coupling between measuring cell CM and the formation studied. The hole is finally filled in with the sampled soil while respecting the vertical zonation thereof. The terminations of tubes TA, TR and of cables CEL are then connected to analysis means MA arranged on the installation surface.

A stage of calibrating the device according to the invention has been carried out prior to the monitoring phase proper. More precisely:

-   -   electrical resistivity measurements are performed by means of a         drainage test by saturating the hole intended to receive         measuring cell CM with fresh water and by measuring the         evolution of the resistivities upon dewatering thereof through         natural drainage and evapotranspiration. Measurements comparable         to those acquired in the laboratory must be obtained. FIG. 7A         (FIG. 7B respectively) shows for example calibration curves for         resistivity R measured by inner electrodes EI (respectively by         outer electrodes EE) as a function of the water saturation SW         for three different assembly configurations of measuring cell         CM,     -   temperature measurements using the temperature probe are checked         by comparison with temperatures recorded at the surface of the         device installation according to the invention,     -   validation of the fluid circulation means is carried out by         injecting a reference gas into reflux tube TR until a         measurement of the reference composition thereof in suction tube         TA is obtained. Injection is then stopped and the fluid         circulation means are restarted in a closed loop. The measured         exchange rates must correspond to those measured in the         laboratory for equivalent water saturations (indicated by the         resistivities).

The measurements performed by means of the device installed in the formation and calibrated are resistivity measurements, temperature measurements and gas analyses. The system is permanently installed and measurements are performed every minute upon gas injection, and every 30 minutes after the injection.

Such a device, made up of fluid-proof elements and comprising fluid circulation means allows the fluid collected in the formation to be refluxed, is suited for long-term monitoring of a formation and, moreover, without measurement perturbation. Besides, the opportunity to perform, in a single measuring cell, different types of physical measurements (electrical, geochemical, temperature) avoids multiple installations on the site and thus allows reduction of installation costs and measurement environment disturbance. Moreover, measurements of various types, performed simultaneously and at a single measuring point, enable more reliable cross-interpretation of the different physical measurement types. 

1-12. (canceled)
 13. A device for monitoring an underground formation containing a fluid comprising at least one measuring cell intended to be arranged in a cavity provided in the underground formation, analysis means intended to be arranged on the surface and connection means for connecting the measuring cell to the analysis means, the cell comprises at least a first chamber, a second chamber impervious to the fluid, and at least three inner connectors sealingly connecting the first chamber to the second chamber; the connection means sealingly connecting the measuring cell to the analysis means, and the connection means comprises sealed protection means; the first chamber comprising a plurality of orifices allowing passage of the fluid into the first chamber, at least two inner electrodes cooperating with at least two of the inner connectors, the inner electrodes being connected to the analysis means through the connection means, the connection means cooperating with the inner connectors, and fluid circulation means connected to the analysis means while cooperating with at least one of the connectors and with the connection means.
 14. A device as claimed in claim 13, wherein the device includes at least two outer electrodes, two inner connectors connecting the two chambers, two sealed outer connectors arranged on at least one wall of the first chamber, the wall being in contact with the formation, the outer electrodes cooperating with the outer connectors, the outer electrodes being connected to the analysis means through the connection means, and the connection means cooperating with the two inner connectors and the two outer connectors.
 15. A device as claimed in claim 13, wherein the connection means includes power supply means supplying the electrodes and the sealed protection means comprise a sealed sheath protecting at least the circulation means and the power supply means.
 16. A device as claimed in claim 14, wherein the connection means includes power supply means supplying the electrodes and the sealed protection means comprise a sealed sheath protecting at least the circulation means and the power supply means.
 17. A device as claimed in claim 13, wherein the first chamber is filled with a permeable porous material having known petrophysical and electrical properties.
 18. A device as claimed in claim 14, wherein the first chamber is filled with a permeable porous material having known petrophysical and electrical properties.
 19. A device as claimed in claim 15, wherein the first chamber is filled with a permeable porous material having known petrophysical and electrical properties.
 20. A device as claimed in claim 16, wherein the first chamber is filled with a permeable porous material having known petrophysical and electrical properties.
 21. A device as claimed in claim 17, wherein the petrophysical properties are at least porosity and permeability, and the electrical properties are at least electrical conductivity.
 22. A device as claimed in claim 13, wherein the connection means, the chambers and the connectors are PTFE.
 23. A device as claimed in claim 14, wherein the connection means, the chambers and the connectors are PTFE.
 24. A device as claimed in claim 15, wherein the connection means, the chambers and the connectors are PTFE.
 25. A device as claimed in claim 16, wherein the connection means, the chambers and the connectors are PTFE.
 26. A device as claimed in claim 17, wherein the connection means, the chambers and the connectors are PTFE.
 27. A device as claimed in claim 13, wherein the analysis means includes at least a fluid analyzer and/or a resistivity meter.
 28. A device as claimed in claim 13, wherein the fluid circulation means includes at least a pipe, a fluid suction system and a fluid reflux system.
 29. A device as claimed in claim 13, wherein the fluid circulation means includes at least two pipes, a fluid suction system and a fluid reflux system.
 30. A device as claimed in claim 13 wherein the underground formation is geological storage site for a gas including either CO₂ or methane.
 31. A device as claimed in claim 30, wherein the device is calibrated prior to injecting gas into geological storage site.
 32. A device in accordance with claim 13 comprising: A device for monitoring an underground formation containing a fluid comprising at least one measuring cell for use in a cavity provided in the underground formation, analysis means for use on the surface and connection means connecting the measuring cell to the analysis means, the cell comprises at least a first chamber, a second chamber impervious to the fluid, and at least three inner connectors sealingly connecting the first chamber to the second chamber, the connection means sealingly connecting the measuring cell to the analysis means, and the connection means comprises sealed protection means, the first chamber comprising: a plurality of orifices allowing passage of the fluid into the first chamber, at least two inner electrodes cooperating with at least two of the inner connectors, the inner electrodes being connected to the analysis means through the connection means, the connection means cooperating with the inner connectors; and fluid circulation means connected to the analysis means while cooperating with at least one of the connectors and with the connection means and wherein the underground cavity in the formation is prepared drilling a cavity which receives the measuring cell of the device; connecting the measuring cell to the connection means of the device, arranging the measuring cell within the cavity and arranging the analysis means of the device on the surface, connecting the analysis means to the connection means, and performing measurements using the measuring cell and analyzing the measurements using the analysis means. 